Infrared radiation detectors convert infrared energy into an electrical signal that can be measured to allow quantification of the amount of infrared radiation received by the detector. One example of the use of such detectors is in the field of infrared spectroscopy, such as Fourier transform infrared (FTIR) spectroscopy. In an FTIR spectrometer a beam of infrared radiation passes through an interferometer, typically a Michelson moving mirror interferometer, and then is transmitted through or reflected from a sample before being detected by the detector. The interferometer can be used to generate a spectrum of the original infrared beam unimpeded by a sample and then a second spectrum with the sample placed in the beam. The difference between the original spectrum and the spectrum obtained with the sample in the beam provides information that can be used to compute the infrared spectral response of the sample.
Certain types of infrared detectors used in FTIR spectrometers are cooled with liquid nitrogen to maintain the detector at a constant low temperature to minimize the effect of thermal noise. Mercury cadium telluride (MCT) detectors are a particular example of sensitive FTIR detectors which are cooled with liquid nitrogen. Cooling of the detector yields particular advantages for infrared analyses, such as spectroscopy, because it improves the signal to noise ratio in the measured electrical signal from the detector and thereby gives greater sensitivity and/or improves spectral resolution.
One type of infrared detector assembly which utilizes cooling with liquid nitrogen is shown in U.S. Pat. No. 4,740,702, the disclosure of which is incorporated herein by reference. This type of detector uses a dewar structure in which there is an inner vessel which contains the liquid nitrogen and which is thermally connected to the detector and an outer vessel which is spaced from and surrounds the inner vessel to provide a space between them which yields thermal isolation of the liquid nitrogen in the inner vessel from ambient atmospheric temperatures.
Detector assemblies and other instruments having dewars that use liquid nitrogen or other liquified gas coolants must be refilled with the cryogenic coolant periodically to replace liquid coolant that has evaporated. In U.S. Pat. No. 4,740,702, a fill port at the top of the assembly provides access to the interior of the liquid nitrogen containment vessel within the dewar. The fill port typically is a small hole on the top surface of the dewar.
In existing designs for FTIR spectrometers, the infrared detector assembly is often positioned deep within the optical section of the instrument. The limited accessibility of this location creates problems when the dewar must be refilled. If the dewar is buried deep within the optical section, access may be extremely limited even with the spectrometer cabinet removed. In addition, there may be drive motors and other mechanical components which actuate the optical elements which may interfere with access to the dewar.
Another problem which can be encountered when the dewar is being refilled is overflow of nitrogen or other cryogenic coolant from the dewar. Not only must the operator be able to pour nitrogen into the dewar, he or she must also be able to determine when the dewar is full, so that liquid nitrogen does not overflow and spill inside the spectrometer. Such spills should be avoided because they can disturb experiments in progress and cause thermal shock that can destroy electronic components. Metal dewars can complicate the task of refilling because the operator may be unable to see adequately inside the fill port to determine when the coolant is about to overflow. Another problem encountered is flow reversals or backflows of the liquid nitrogen. As liquid nitrogen evaporates, the gas at atmospheric pressure occupies about 650 times the space of the corresponding liquified nitrogen. Many dewar designs have a relatively small fill port, and if the operator attempts to pour liquid nitrogen into the fill port too quickly, the nitrogen evaporating inside the dewar may create enough back pressure to reverse the flow of liquid nitrogen being poured in. In such cases, liquid nitrogen can spew out of the fill port. Thus, it is desirable to provide an easy escape for the large volume of nitrogen gas that evaporates inside the containment vessel in the dewar.